Retina implants of various kinds are well-known in the prior art. For example, European published patent application 0 460 320 discloses a subretinal implant to be implanted between lower layers of the retina. This prior art implant essentially consists of a silicon chip configured by a large number of densely packed microphotodiodes. The photoactive surface of the photodiodes is directed to the light impinging through the eyes on the retina. The photodiodes generate an amplitude modulated current stimulating the cellular layer of the retina lying on the implant surface. By using such implant it shall be possible to enable patients suffering from retinal degenerations to improve or even regain vision.
The prior art implant is configured such that the impinging ambient light shall be sufficient to generate the required stimuli for the retinal cells. Hence, an external energy supply is not provided.
In the scientific journal IEEE Spectrum of May 1996, pp. 47-53, still another retina implant is disclosed which, however, shall be used as an epiretinal implant. Consequently, this prior art implant has an electrode surface which is placed between the vitreous body of the eye and the retina surface for stimulating ganglion cells located just below the retina surface.
In contrast to the situation with subretinal implants, when epiretinal implants are used, it is necessary to encode the stimuli in order to compensate for the biological variations to which the biological signals are subjected on their way between the photoreceptors and the ganglion cells in the lower retinal layers. Epiretinal implants are, therefore, controlled externally, namely on the basis of an image which, for example, is generated by a video camera or the like which, in turn, may be configured as spectacles for a patient.
in view of these systematic distinctions between subretinal implants and epiretinal implants, the latter are larger in view of their specific design, are more complicated and have a higher energy consumption.
In the prior art epiretinal implant mentioned before, the image viewed by the patient is recorded by means of a CCD-camera which, together with a signal processing chip and a laser are provided in a spectacle-like device.
Within the implant itself the electrode array that is provided for stimulating the ganglions, is configured as a thin blade extending laterally from the implant as such. The blade is placed on the retina. The implant body as such is configured relatively bulky. Further to a stimulator chip it comprises a photodiode array being directed against the eye opening. The photodiode array receives a light signal from the laser being arranged in the spectacle-like device. The light signal is within the visible portion of the spectrum and contains the image signals on the one hand side and, further, light energy used for the implant energy supply.
It is, therefore, a disadvantage of this prior art epiretinal implant that a considerable portion of the surface in the area of the retina must be obscured by the photodiode array in order to ensure the necessary signal transmission and power supply. That portion of the retina which is obscured by the implant may, hence, not used for stimulation. Further, the amount of energy that may be transmitted into the eye is limited by the physiological threshold of the retina with respect to visible light.
In a subretinal implant of the kind mentioned at the outset, the problem is that ambient light may be insufficient to generate stimuli of sufficient amplitude being larger than the stimulus threshold of the retinal cells to be stimulated.
It is, therefore, an object underlying the invention to improve an implant of the kind mentioned at the outset such that even under poor ambient light conditions there is sufficient energy available to generate stimuli of required amplitude. When doing so the disadvantages shall be avoided associated to epiretinal implants of the kind discussed above and consisting mainly in that certain areas of the retina may not be used for stimulation because they are obscured by implant components.